The electroluminescent element (hereinafter referred to as EL element) which emits light when energized by an AC field has a laminate structure consisting of a thin film electroluminor or phosphor layer, a thin film dielectric layer or layers provided on one or both sides of said phosphor layer and two electrode layers holding said layers. The phosphor layer used in such EL element is basically composed of such material as ZnS, ZnSe or ZnF.sub.2 in which Mn or a rare earth fluoride is added as luminescent center. For instance, a phosphor layer composed of ZnS and having Mn added as a luminescent center is capable of providing a luminance of 3,500 to 5,000 Cd/m.sup.2 at most with application of an AC voltage of 5 KHz. As the dielectric material, Y.sub.2 O.sub.3, SiO.sub.2, Si.sub.3 N.sub.4, Al.sub.2 O.sub.3 and Ta.sub.2 O.sub.5 are mostly used. As for the layer thickness, usually the ZnS layer is of a thickness in the range of 5,000 to 7,000 .ANG. and the dielectric layer thickness is in the range of 4,000 to 8,000 .ANG..
In the case of AC drive, the voltage applied to the element is divided between the ZnS layer and the dielectric layer. Since the EL element is equivalent to two series-connected capacitors, the relation of .epsilon..sub.i V.sub.i /t.sub.i =.epsilon..sub.z V.sub.z /t.sub.z (where .epsilon. is the dielectric constant, V is applied voltage, t is thickness, i is dielectric and z is ZnS) holds, and in view of this relation, each partial voltage is found inversely proportional to the dielectric constant if t.sub.i =t.sub.z. Therefore, since .epsilon..sub.i of a dielectric such as Y.sub.2 O.sub.3 is about 4 to 25 and .epsilon..sub.z of ZnS is about 9, only 30 to 70% of the whole applied voltage is given to the ZnS layer. Thus, in such element, a voltage of higher than 200 V needs to be applied for pulse drive at several KHz. Such high voltage puts too much load on the drive circuits and necessitates the use of a specific high voltage resistant drive IC, which leads to an elevated production cost. Naturally, an element which can be driven at as low a voltage as possible and which is high in production yield and excellent in quality and reliability has been desired in the industries.
For lowering the drive voltage, the dielectric layer is required to have specific characteristics that are discussed below. In view of the relation of voltage division shown above, it is understood that .epsilon..sub.i /t.sub.i must be large. After the start of light emission, the increment of applied voltage is exclusively applied to the dielectric layer, so that it is an essential requirement for an excellent dielectric film to have a large value of V.sub.ib (dielectric breakdown voltage). Therefore, the figure of merit .gamma. of the dielectric film is expressed by: .gamma.=.epsilon..sub.i V.sub.ib /t.sub.i =.epsilon..sub.i E.sub.ib (wherein E.sub.ib is dielectric breakdown field strength of the dielectric film). As seen from the above equation, .gamma. is proportional to the electric charges accumulated per unit area at the time of dielectric breakdown of the dielectric film. The greater .gamma., the more a stable low-voltage drive becomes possible. Here, let it be supposed that two EL elements have been produced in which the thickness of both phosphor layer and dielectric layer are the same, and also suppose that one of the elements has a dielectric film of the following charactersitics: .epsilon..sub.i =100, E.sub.ib =1.times.10.sup.6 V/cm, and .gamma.=100.times.10.sup.6 V/cm and the other element has the dielectric characteristics of .epsilon..sub.i =50, E.sub.ib =3.times.10.sup.6 V/cm and .gamma.=150.times.10.sup.6 V/cm, then naturally, the former element, where .epsilon..sub.i =100, can emit light at a lower voltage since both elements have the same dielectric thickness. On the other hand, in the case of the latter element where .epsilon..sub.i =50 and E.sub.ib =3.times.10.sup.6 V/cm, which is higher in dielectric breakdown voltage, the film thickness can be reduced to 1/3 of the former element if both elements are equal in dielectric strength. Accordingly, the capacity of the dielectric is tripled, making .epsilon..sub.i =150 equivalently. Therefore, a higher figure of merit makes it possible to obtain an element that can emit light at a lower voltage, regardless of .epsilon..sub.i. It is desirable that the value of .gamma. be as large as possible. More specifically, as a measure of low-voltage light emission, it is suggested that .gamma. be more than 10 times the value of 14.times.10.sup.6 V/cm obtained by substituting .epsilon..sub.z =9 and E.sub.zb =1.6.times.10.sup.6 V/cm of ZnS for .epsilon..sub.i and E.sub.ib in the above-shown equation.
The figure of merit of the conventional dielectric films is of the order of 50.times.10.sup.6 V/cm in the case of Y.sub.2 O.sub.3, 30.times.10.sup.6 V/cm in the case of Al.sub.2 O.sub.3 and 70.times.10.sup.6 V/cm in the case of Si.sub.3 N.sub.4. These values are too small for realizing low-voltage light emission.
More recently, use of thin films mainly composed of PbTiO.sub.3 or Pb(Ti.sub.1-x Zr.sub.x)O.sub.3 with a high dielectric constant for the dielectric layer has been proposed. In these films, .epsilon..sub.i can be over 150 but on the other hand E.sub.ib is as small as 0.5-0.6.times.10.sup.6 V/cm, so that it is necessary to greatly increase the film thickness as compared with films using conventional dielectric materials. Therefore, in view of practical reliability of the element, it is required that said dielectric film has a thickness greater than 15,000 .ANG., for up to 6,000 .ANG. in thickness of the ZnS film. Generally, in the use of such material, the grains in the film tend to grow causing cloudiness because of high substrate temperature at the time of film formation in addition to large film thickness. In an X,Y matrix display using such cloudy films, light is let out from the non-excited segments because the light emitted from excited segments is scattered, resulting in a degraded image quality.
In view of the above, the present inventors have proposed an EL element using a dielectric film mainly composed of SrTiO.sub.3, which film is high in both E.sub.ib and E.sub.ib .times..epsilon..sub.i, suited for low-voltage drive and free of clouding.
Reduction of drive voltage is desirable from the viewpoints of reliability and cost of the drive circuits, but no technical criteria has been made on this matter. It has therefore been required to conduct further research on said SrTiO.sub.3 dielectric film from its compositional aspect and to obtain the improved characteristics.